DR400 requires a min of ISO 800 and DR200 requires a min of ISO 400 to work. The extended DR is to prevent blowing out highlights. The way it does this is by exposing one or two stops under to capture highlight detial, then it compensates in the midtone and shadow by increasing signal via the ISO. This is why it needs that extra stop, ISO200, or two, ISO400, to work. By setting DR to 400 you forced the camera to use the higher ISO. This feature can be extremely useful and is designed exclusively to add highlight detail, so it is not useful if there are no bright areas to extend the Range. It is to pull back highlights.

See below how I needed it to not blow out the lights or the skating rink. These are essentially straight out of the camera at DR400.